Pressure-activated electrical interconnection by micro-transfer printing

ABSTRACT

A printed electrical connection structure includes a substrate having one or more electrical connection pads and a micro-transfer printed component having one or more connection posts. Each connection post is in electrical contact with a connection pad. A resin is disposed between and in contact with the substrate and the component. The resin has a reflow temperature less than a cure temperature. The resin repeatedly flows at the reflow temperature when temperature-cycled between an operating temperature and the reflow temperature but does not flow after the resin is exposed to a cure temperature. A solder can be disposed on the connection post or the connection pad. After printing and reflow, the component can be tested and, if the component fails, another component is micro-transfer printed to the substrate, the resin is reflowed again, the other component is tested and, if it passes the test, the resin is finally cured.

PRIORITY APPLICATION

This application claims priority to and benefit of U.S. PatentApplication No. 62/317,107, filed Apr. 1, 2016, entitledPressure-Activated Electrical Interconnection by Micro-TransferPrinting, the content of which is hereby incorporated by reference inits entirety.

CROSS REFERENCE TO RELATED APPLICATION

Reference is made to U.S. Pat. No. 8,889,485, entitled Methods forSurface Attachment of Flipped Active Components by Christopher Bower, toU.S. patent application Ser. No. 14/822,864, entitled Chiplets withConnection Posts by Prevatte et al, to U.S. patent application Ser. No.14/743,788, entitled Micro Assembled LED Displays and Lighting Elementsby Bower et al., and to U.S. patent application Ser. No. 15/373,865,entitled Micro-Transfer Printable Electronic Component by Cok et al.,the disclosures of which are incorporated herein by reference in theirentirety.

FIELD OF THE INVENTION

The present invention relates to structures and methods for electricallyinterconnecting chiplets to backplane electrical contact pads usingmicro transfer printing.

BACKGROUND OF THE INVENTION

Substrates with electronically active components distributed over theextent of the substrate may be used in a variety of electronic systems,for example, flat-panel imaging devices such as flat-panel liquidcrystal or organic light emitting diode (OLED) display devices and inflat-panel solar cells. A variety of methods may be used to distributeelectronically active circuits over substrates, including forming theelectronically active circuits on a substrate and forming the componentson separate substrates and placing them on a substrate. In the lattercase, a variety of assembly technologies for device packaging may beused.

Electronically active components are typically formed on a substrate bysputtering a layer of inorganic semiconductor material or byspin-coating organic material over the entire substrate. Inorganicsemiconductor materials can be processed to improve their electroniccharacteristics, for example amorphous silicon can be treated to formlow-temperature or high-temperature poly-crystalline silicon. In otherprocess methods, microcrystalline semiconductor layers can be formed byusing an underlying seeding layer. These methods typically improve theelectron mobility of the semiconductor layer. The substrate and layer ofsemiconductor material can be photo-lithographically processed to defineelectronically active components, such as transistors. Such transistorsare known as thin-film transistors (TFTs) since they are formed in athin layer of semiconductor material, typically silicon. Transistors mayalso be formed in thin layers of organic materials. In these devices,the substrate is often made of glass, for example Corning Eagle® orJade® glass designed for display applications.

The above techniques have some limitations. Despite processing methodsused to improve the performance of thin-film transistors, suchtransistors may provide performance that is lower than the performanceof other integrated circuits formed in mono-crystalline semiconductormaterial. Semiconductor material and active components can be providedonly on portions of the substrate, leading to wasted material andprocessing costs. The choice of substrate materials can also be limitedby the processing steps necessary to process the semiconductor materialand the photo-lithographic steps used to pattern the active components.For example, plastic substrates have a limited chemical and heattolerance and do not readily survive photo-lithographic processing.Furthermore, the manufacturing equipment used to process largesubstrates with thin-film circuitry is relatively expensive. Othersubstrate materials that may be used include quartz, for example, forintegrated circuits using silicon-on-insulator structures as describedin U.S. Patent Application No. 2010/0289115 and U.S. Patent ApplicationNo. 2010/0123134. However, such substrate materials can be moreexpensive or difficult to process.

Other methods used for distributing electronically functional componentsover a substrate in the circuit board assembly industry includepick-and-place technologies for integrated circuits provided in avariety of packages, for example, pin-grid arrays, ball-grid arrays, andflip-chips. However, these techniques may be limited in the size of theintegrated circuits that can be placed.

In other manufacturing techniques, a mono-crystalline semiconductorwafer is employed as the substrate. While this approach can providesubstrates with the same performance as integrated circuits, the size ofsuch substrates may be limited, for example, to a 12-inch diametercircle, and the wafers are relatively expensive compared to othersubstrate materials such as glass, polymer, or quartz.

In yet another approach, thin layers of semiconductor are bonded to asubstrate and then processed. Such a method is known assemiconductor-on-glass or silicon-on-glass (SOG) and is described, forexample, in U.S. Pat. No. 7,605,053, issued Oct. 20, 2009. If thesemiconductor material is crystalline, high-performance thin-filmcircuits can be obtained. However, the bonding technique and theprocessing equipment for the substrates to form the thin-film activecomponents on large substrates can be relatively expensive.

Publication No. 11-142878 of the patent Abstracts of Japan entitledFormation of Display Transistor Array Panel describes etching asubstrate to remove it from a thin-film transistor array on which theTFT array was formed. TFT circuits formed on a first substrate can betransferred to a second substrate by adhering the first substrate andthe TFTs to the surface of the second substrate and then etching awaythe first substrate, leaving the TFTs bonded to the second substrate.This method may require etching a significant quantity of material, andmay risk damaging the exposed TFT array.

Other methods for locating material on a substrate are described in U.S.Pat. No. 7,127,810. In this approach, a first substrate carries athin-film object to be transferred to a second substrate. An adhesive isapplied to the object to be transferred or to the second substrate inthe desired location of the object. The substrates are aligned andbrought into contact. A laser beam irradiates the object to abrade thetransferring thin film so that the transferring thin film adheres to thesecond substrate. The first and second substrates are separated, peelingthe film in the abraded areas from the first substrate and transferringit to the second substrate. In one embodiment, a plurality of objects isselectively transferred by employing a plurality of laser beams toabrade selected area. Objects to be transferred can include thin-filmcircuits.

U.S. Pat. No. 6,969,624 describes a method of transferring a device froma first substrate onto a holding substrate by selectively irradiating aninterface with an energy beam. The interface is located between a devicefor transfer and the first substrate and includes a material thatgenerates ablation upon irradiation, thereby releasing the device fromthe substrate. For example, a light-emitting device (LED) is made of anitride semiconductor on a sapphire substrate. The energy beam isdirected to the interface between the sapphire substrate and the nitridesemiconductor releasing the LED and allowing the LED to adhere to aholding substrate coated with an adhesive. The adhesive is then cured.These methods, however, may require the patterned deposition of adhesiveon the object(s) or on the second substrate. Moreover, the laser beamthat irradiates the object may need to be shaped to match the shape ofthe object, and the laser abrasion can damage the object to betransferred. Furthermore, the adhesive cure takes time, which may reducethe throughput of the manufacturing system.

Micro-transfer-printing is an advanced assembly technology forapplications that benefit from heterogeneous integration ofhigh-performance micro-scale devices. Micro-device systems compatiblewith micro-transfer-printing include silicon integrated circuits, solarcells, light emitting diodes, compound semiconductor transistors, andlasers.

In micro-transfer-printing, engineered viscoelastic elastomer stamps areused to pick up and transfer arrays of components from the nativesubstrate on or in which the components are formed onto non-nativedestination substrates. The components are fabricated using maturematerials and processes, and are made print-compatible usingmicromachining or etching processes which leave the micro-componentsundercut. The undercut components remain fixed to the native waferthrough tethering structures connected to non-undercut anchors.Conventional photolithographic methods are then used to form thin-filmmetal traces which interconnect the printed device arrays.

Such a micro-transfer printing method for transferring active componentsfrom one substrate to another is described in AMOLED Displays usingTransfer-Printed Integrated Circuits published in the Proceedings of the2009 Society for Information Display International Symposium Jun. 2-5,2009, in San Antonio Tex., US, vol. 40, Book 2, ISSN 0009-0966X, paper63.2 p. 947. In this approach, small integrated circuits are formed overa buried oxide layer on the process side of a crystalline wafer. Thesmall integrated circuits, or chiplets, are released from the wafer byetching the buried oxide layer formed beneath the circuits. A PDMS stampis pressed against the wafer and the process side of the chiplets isadhered to the stamp. The chiplets are pressed against a destinationsubstrate or backplane coated with an adhesive and thereby adhered tothe destination substrate. The adhesive is subsequently cured. Inanother example, U.S. Pat. No. 8,722,458 entitled Optical SystemsFabricated by Printing-Based Assembly teaches transferringlight-emitting, light-sensing, or light-collecting semiconductorelements from a wafer substrate to a destination substrate or backplane.

In such system it is necessary to electrically connect the smallintegrated circuits or chiplets to electrically conductive elements suchas backplane contact pads on the destination substrate. By applyingelectrical signals to conductors on the destination substrate the smallintegrated circuits are energized and made operational. The electricalconnections between the small integrated circuits and the backplanecontact pads are typically made by photolithographic processes in whicha metal is evaporated or sputtered onto the small integrated circuitsand the destination substrate to form a metal layer, the metal layer iscoated with a photoresist that is exposed to a circuit connectionpattern, and the metal layer and photoresist are developed by etchingand washing to form the patterned electrical connections between thesmall integrated circuits and the connection pads on the destinationsubstrate. Additional layers, such as interlayer dielectric insulatorscan also be required. This process is expensive and requires a number ofmanufacturing steps. Moreover, the topographical structure of the smallintegrated circuits over the destination substrate renders theelectrical connections problematic. For example, it can be difficult toform a continuous conductor from the destination substrate to the smallintegrated circuit because of the differences in height over the surfacebetween the small integrated circuits and the destination substrate.

There is a need, therefore, for structures and methods that enable theelectrical interconnection of small integrated circuits, such asmicro-transfer printed chiplets, to destination substrates.

In one aspect, the disclosed technology includes a printed electricalconnection structure, including: a substrate having one or moreelectrical connection pads; a printed component having one or moreconnection posts, each connection post in electrical contact with aconnection pad; and a resin disposed between and in contact with thesubstrate and the printed component, the resin having a reflowtemperature less than a cure temperature, wherein the resin repeatedlyflows at the reflow temperature when temperature cycled between anoperating temperature and the reflow temperature but does not flow afterthe resin is exposed to the cure temperature.

In certain embodiments, the printed structure includes a solder disposedon the connection post, on the connection pad or on both the connectionpost and the connection pad.

In certain embodiments, the solder is in electrical contact with boththe connection post and the connection pad.

In certain embodiments, the solder has a melting temperature between thereflow temperature and the cure temperature.

In certain embodiments, the solder has a melting temperature less thanor equal to the reflow temperature.

In certain embodiments, the solder has a melting temperature greaterthan or equal to the cure temperature.

In certain embodiments, the connection pad includes metal and the solderis softer than the connection pad.

In certain embodiments, the printed structure includes a plurality ofconnection posts and connection pads each connection post electricallyconnected to at least one connection pad.

In certain embodiments, the resin is at least partly between two or moreconnection posts, substantially fills the volume between the printedcomponent and the substrate defined by the area defined or bounded bythe two or more connection posts, or wherein the resin has a greaterthickness between the printed component and the substrate than at leastsome other areas of the substrate.

In certain embodiments, the printed component is a first printedcomponent, the one or more connection pads are one or more firstconnection pads, and comprising a second printed component having one ormore second connection posts, each second connection post in electricalcontact with the second connection pads, the resin is disposed betweenand in contact with the substrate and the second printed component, andthe first and second connection pads are electrically connected so thatthe first and second components are electrically connected in parallel.

In certain embodiments, the connection pad and the connection post bothinclude metal and the metal of the connection pad has a differenthardness than the metal of the connection post.

In another aspect, the disclosed technology includes a method of makinga printed structure, including: providing a substrate having one or moreelectrical connection pads; disposing a resin layer over and in contactwith at least a portion of the substrate; and micro-transfer printing acomponent from a component source wafer to the substrate, the componenthaving one or more connection posts that are each positioned adjacent toa corresponding connection pad, so that the resin contacts at least aportion of the printed component.

In certain embodiments, the resin has a reflow temperature less than acure temperature, wherein the resin repeatedly flows at a first rate atthe reflow temperature when temperature cycled between an operatingtemperature and the reflow temperature but does not flow after the resinis exposed to a cure temperature.

In certain embodiments, the method includes testing the component.

In certain embodiments, the method includes heating the resin to thereflow temperature and electrically contacting each connection post tothe corresponding connection pad.

In certain embodiments, the method includes cooling the structure afterheating the resin to the reflow temperature.

In certain embodiments, the method includes reheating the structure tothe reflow temperature after cooling the structure.

In certain embodiments, the method includes cooling the structure afterreheating the resin to the reflow temperature.

In certain embodiments, the method includes heating the structure to thecure temperature after reheating the resin to the reflow temperature.

In certain embodiments, the method includes testing the structure afterreheating the resin to the reflow temperature.

In certain embodiments, the method includes testing the structure afterheating the resin to the reflow temperature.

In certain embodiments, the method includes providing a layer of solderover at least the one or more connection pads, the solder having amelting temperature.

In certain embodiments, the method includes heating the structure to themelting temperature.

In certain embodiments, the method includes heating the structure to themelting temperature after heating the structure to the reflowtemperature.

In certain embodiments, the method includes heating the structure to themelting temperature after heating the structure to the cure temperature.

In certain embodiments, the method includes providing an electricalconductor electrically connected to one of the connection pads, andcutting the electrical conductor.

In certain embodiments, the component is a first component, the one ormore connection pads are one or more first connection pads, andcomprising providing one or more second connection pads electricallyconnected in parallel with the one or more first connection pads.

In certain embodiments, the method includes determining that the firstcomponent failed the test.

In certain embodiments, the method includes providing an electricalconductor electrically connected to one of the connection pads, andcutting the electrical conductor.

In certain embodiments, the method includes micro-transfer printing asecond component from a source wafer to the substrate, the secondcomponent having one or more second connection posts that are eachadjacent to a corresponding second connection pad, and the resin iscontacted to at least a portion of the second printed component.

In certain embodiments, the method includes heating the resin to thereflow temperature and electrically contacting each second connectionpost to the corresponding second connection pad.

In certain embodiments, the method includes heating the structure to thereflow temperature, cooling the structure, testing the structure,reheating the structure to the reflow temperature, cooling thestructure, and heating the structure to the cure temperature.

In certain embodiments, the component is a first component, the one ormore connection pads are one or more first connection pads, comprisingproviding one or more second connection pads electrically connected inparallel with the one or more first connection pads, determining thefirst component failed the test, micro-transfer printing a secondcomponent from a source wafer to the substrate, the second componenthaving one or more second connection posts that are each adjacent to acorresponding second connection pad, and the resin is contacted to atleast a portion of the second printed component.

In certain embodiments, the method includes testing the secondcomponent.

In another aspect, the disclosed technology includes a micro-transferprintable component, including: a dielectric substrate having a postside and a circuit side; one or more electrically conductive connectionposts protruding from the post side of the dielectric substrate; acircuit disposed on the circuit side of the dielectric substrate; and anelectrode electrically connecting each of the connection posts to thecircuit.

In certain embodiments, the micro-transfer printable component includesa via corresponding to each connection post, the via extending from thecircuit side of the dielectric substrate to a portion of thecorresponding connection post, the electrode extending into the via toelectrically connect the circuit to the corresponding connection post.

In certain embodiments, the circuit is an LED.

In certain embodiments, the LED includes electrical contacts located ona common side of the LED opposite the dielectric substrate.

In certain embodiments, the LED includes electrical contacts located ona common side of the LED adjacent to the dielectric substrate.

In certain embodiments, the LED emits light through the dielectricsubstrate.

In certain embodiments, the LED emits light in a direction opposite thedielectric substrate.

In another aspect, the disclosed technology includes a method of makinga micro-transfer printable component, includes: providing a sacrificiallayer on a source wafer;

etching one or more structures into the sacrificial layer; patterning aconnection post in each etched structure; depositing a dielectricsubstrate over the sacrificial layer and each connection post; forming avia in the dielectric substrate over a portion of each correspondingconnection post; disposing a circuit on or in the dielectric substrate;and patterning one or more electrodes electrically connecting thecircuit to one or more of the connection posts over the dielectricsubstrate and through the corresponding via to form the micro-transferprintable component.

In certain embodiments, the method includes disposing the circuit bymicro-transfer printing the circuit from a circuit source wafer to thedielectric substrate.

In certain embodiments, the method includes etching the sacrificiallayer to release the micro-transfer printable component from the sourcewafer.

In certain embodiments, the method includes micro-transfer printing themicro-transfer printable component to a destination substrate.

In certain embodiments, the method includes forming a tether by etchingthe sacrificial layer and fracturing the tether by micro-transferprinting the micro-transfer printable component.

In certain embodiments, the circuit is an LED.

In certain embodiments, the method includes a fractured tether connectedto the dielectric substrate.

In certain embodiments, the fractured tether is connected to thedielectric substrate, is a part of the dielectric substrate, or includesthe same material as the dielectric substrate.

In certain embodiments, the method includes an encapsulation layerhaving a thickness that is less than the dielectric substrate andwherein the fractured tether is connected to the encapsulation layer,includes the same material as the dielectric substrate, or is a part ofthe dielectric substrate, and the tether has a thickness less than thedielectric layer thickness.

SUMMARY OF THE INVENTION

In accordance with embodiments of the present invention, components suchas chiplets incorporating active elements such as transistors or passiveelements such as resistors, capacitors, and conductors aremicro-transfer printed from a native source wafer to a non-nativedestination substrate or backplane. In various embodiment, thecomponents are LEDs, electrical connection jumpers, or integrated logiccircuits, or a combination of such elements. The components include anelectrically conducting connection post that protrudes in a directionaway from a component surface and is brought into contact with aconnection pad on a destination substrate to form an electricalconnection between the component and the destination substrate. Thecomponents can be at least partially adhered to the destinationsubstrate by forcefully driving the connection posts into the substrateconnection pads when micro-transfer printing, for example by exertingmechanical pressure on the transfer stamp.

The connection posts, the substrate connection pads, or both theconnection posts and the substrate connection pads can be deformed orcrumpled and the connection post can be driven into or through thesubstrate connection pad, thereby wedging the connection post in thesubstrate connection pad to adhere the connection post to the substrateconnection pad and form an electrical contact between them. As aconsequence, the connection post can be welded to the substrateconnection pad. An additional heat treatment can be provided tofacilitate the welding. Alternatively or additionally, a layer of metal,for example a solder can be provided on either the surface of theconnection post or the substrate connection pad, or both, that can beheated, causing the solder to reflow and thereby both adhere andelectrically connect the connection post to the substrate connectionpad.

In an embodiment of the present invention, an adhesive layer adheres thecomponent to the destination substrate and facilitates an electricalconnection between the connection post and the connection pad. In thisembodiment, a printed electrical connection structure includes asubstrate having one or more electrical connection pads. A printedcomponent has one or more connection posts and each connection post isin electrical contact with a connection pad. A resin is disposed betweenand in contact with the substrate and the printed component. The resinhas an operating temperature that is less than a reflow temperature andthe reflow temperature is less than a cure temperature. Until the resinis brought to the cure temperature, it can flow at the reflowtemperature so that the resin repeatedly flows at the reflow temperaturewhen temperature-cycled between an operating temperature and the reflowtemperature but does not flow after the resin is exposed to a curetemperature. Thus, the resin does not substantially flow at theoperating temperature but does flow at the reflow temperature. The resincan be repeatedly temperature-cycled between the two states. When thetemperature is raised to a cure temperature greater than the reflowtemperature, however, the resin cures and will no longer flow at thereflow temperature.

In another embodiment of the present invention, two or more connectionposts are provided to contact a common connection pad. By providing twoor more connection posts in contact with a common connection pad, faultsin electrical connections between the component and the connection padare reduced by providing a redundant electrical connection from thecomponent to the connection pad.

In a further embodiment, a solder is disposed on the connection post,the connection pad, or both, and is in electrical contact with both theconnection post and the connection pad. The solder can have a meltingtemperature and can flow at a temperature less than or equal to thereflow temperature, a temperature between the reflow temperature and thecure temperature, or a temperature greater than or equal to the curetemperature. The solder can be softer than a material in the connectionpad or in the connection post and the connection post can have adifferent hardness than the connection pad.

A method of making a printed structure includes providing a substratehaving one or more electrical connection pads, disposing a patterned orunpatterned resin layer over and in contact with at least a portion ofthe substrate, and micro-transfer printing a component from a sourcewafer to the substrate. The component has one or more connection poststhat are each positioned adjacent to or in contact with a correspondingconnection pad and the resin is contacted to at least a portion of theprinted component. The printed structure is heated to the reflowtemperature to reflow the resin, cooled to an operating temperature, andthen tested. If the test is passed, the printed structure is heated tothe cure temperature and the process is complete. If the test is failed,another component is printed to the substrate, for example bymicro-transfer printing the other component to connection pads connectedelectrically in parallel with the failed component. The printedstructure is reheated to the reflow temperature to reflow the resin,cooled to an operating temperature, and then tested again. If the othercomponent passes the test, the printed structure is heated to the curetemperature and the process is complete. If not, the process isrepeated.

The resin can be disposed over the substrate and connection pads in anunpatterned layer, for example by coating or lamination. Alternatively,the resin can be disposed in a pattern, for example over the connectionpads or between the connection pads, for example using inkjet orphotolithographic techniques.

In an embodiment, the connection pads are not electrically connected tothe connection posts immediately following micro-transfer printing thecomponent to the substrate. By heating the structure to the reflowtemperature, the resin flows and wicks over the substrate, theconnection pads, the connection posts, and optionally the component.This reduces the volume between the component and the substrate, drawingthe component closer to the substrate and, in particular, embedding theconnection posts into the connection pads and forming or improving anelectrical connection between the connection posts and the connectionpads. Subsequent reflow operations do not undo the electrical connectionand can even improve it. Thus, if two components are micro-transferprinted to a substrate, the resin for both reflowed, only one componentfails the test, a new component is micro-transfer printed electricallyin parallel with the failed component, and the resin for reflowed again,the originally functional component remains functional and electricallyconnected to its corresponding connection pads.

In a further embodiment, heating the resin to the reflow temperature orcure temperature melts a solder that further facilitates the electricalconnection between the connection posts and the connection pads.

In a further embodiment of the present invention, a defective componentis removed from the substrate and replaced, for example by microtransfer printing another component to the substrate in the formerlocation of the removed defective chiplet.

In another embodiment, a micro-transfer printable component includes adielectric substrate having a post side and a circuit side and one ormore electrically conductive connection posts protruding from the postside of the dielectric substrate. A circuit is disposed on the circuitside of the dielectric substrate and an electrode electricallyconnecting each of the connection posts to the circuit. A viacorresponding to each connection post can extend from the circuit sideof the dielectric substrate to a portion of the corresponding connectionpost and the electrode can extend into the via to electrically connectthe circuit to the corresponding connection post. The circuit can be anLED and the LED can include electrical contacts located on a common sideof the LED opposite the dielectric substrate.

Because the components can be made using integrated circuitphotolithographic techniques having a relatively high resolution andcost and the destination substrate, for example a printed circuit board,can be made using printed circuit board techniques having a relativelylow resolution and cost, the connection pads on the destinationsubstrate can be much larger than the connection posts or electricalcontacts on the component, reducing manufacturing costs.

In one aspect, the disclosed technology includes a printable componentincluding a chiplet having a semiconductor substrate and a plurality ofelectrical connections, wherein each electrical connection comprises anelectrically conductive connection post protruding from thesemiconductor substrate or a plane parallel to the semiconductorsubstrate. In an embodiment, the connection post is a multi-layerconnection post. In certain embodiments, the printable component is anactive component having an active element, a passive component having apassive element, or a compound structure having a plurality of activeelements, passive elements, or a combination of active and passiveelements.

In certain embodiments, the printable component has at least one of awidth, length, and height from 2 to 5 μm, 5 to 10 μm, 10 to 20 μm, or 20to 50 μm.

In certain embodiments, the printable component is or includes alight-emitting diode, photo-diode, or transistor.

In another aspect, the disclosed technology includes a printedelectrical connection structure comprising a destination substrate andone or more printable components having connection posts, wherein thedestination substrate has two or more electrical connection pads andeach connection post is in electrical contact with, extends into, orextends through an electrical connection pad of the destinationsubstrate to electrically connect the electrical pads to the connectionposts.

In certain embodiments, the electrical contact comprises a material thatis the same material as a material included in the connection post.

In certain embodiments, the destination substrate is a member selectedfrom the group consisting of polymer, plastic, resin, polyimide, PEN,PET, metal, metal foil, glass, a semiconductor, and sapphire. In certainembodiments, the destination substrate has a thickness from 5 to 10microns, 10 to 50 microns, 50 to 100 microns, 100 to 200 microns, 200 to500 microns, 500 microns to 0.5 mm, 0.5 to 1 mm, 1 mm to 5 mm, 5 mm to10 mm, or 10 mm to 20 mm.

In certain embodiments, a conductive material other than a material ofthe substrate connection pad or the connection post adheres orelectrically connects (e.g., or both) the substrate connection pad tothe connection post. In certain embodiments, the substrate connectionpad has a first conductive layer and a second conductive layer over thefirst conductive layer, and the second conductive layer has a lowermelting temperature than the first conductive layer, wherein thesubstrate connection pad is coated with a non-conductive layer, orwherein the substrate connection pad is formed on a compliantnon-conductive layer. In certain embodiments, the second conductivelayer is a solder or a conductive polymer. In certain embodiments, thesecond conductive layer is a non-conductive adhesive or a resin.

In certain embodiments, the connection pad is welded to the connectionpost. In certain embodiments, the substrate connection pads arenon-planar and the connection posts are inserted into the backplanecontact pads.

In an embodiment, a method of making a micro-transfer printablecomponent includes providing a sacrificial layer on a source wafer andetching one or more structures into the sacrificial layer. A connectionpost is patterned in each etched structure and a dielectric substratedeposited, laminated, or otherwise disposed over the sacrificial layerand each connection post. A via is formed in the dielectric substrateover a portion of each corresponding connection post and a circuitdisposed on or in the dielectric substrate. One or more electrodeselectrically connecting the circuit to one or more of the connectionposts are patterned over the dielectric substrate and through thecorresponding via. The circuit can be micro-transfer printing from acircuit source wafer to the dielectric substrate. The sacrificial layercan be etched to release the micro-transfer printable component from thesource wafer and the micro-transfer printable component micro-transferprinted to a destination substrate. The circuit can be an LED.

In another aspect, the disclosed technology includes a method of makinga printable component, including: providing a forming substrate havingtwo or more forms in a surface of the substrate; disposing a patternedlayer of conductive material at least in the forms to make connectionposts; disposing a first dielectric layer over the patterned layer ofconductive material and the forming substrate; disposing a chiplethaving chiplet contact pads on the first dielectric layer; formingconductors electrically connecting the connection posts to the chipletcontact pads; and defining the printable component to form a releaselayer and anchors in the forming substrate connected by tethers to theprintable component. The first dielectric layer can be transparent.

In certain embodiments, the method includes providing a destinationsubstrate having two or more substrate connection pads; and microtransfer printing the printable component to the destination substrateso that each connection post is in contact with, extends into, orextends through a substrate connection pad of the destination substrateto electrically connect the substrate connection pads to the connectionposts and the printed component.

In certain embodiments, the method includes disposing a patterned seconddielectric layer disposed at least partly over the first dielectriclayer, the conductors, and the chiplet. In certain embodiments, thesecond dielectric layer is transparent, and the component is alight-emitting component that emits light through the second dielectriclayer.

In another aspect, the disclosed technology includes a printablecomponent, including: a first dielectric layer having connection postsprotruding from the dielectric layer; a chiplet having a semiconductorsubstrate and chiplet contact pads, the chiplet disposed on the firstdielectric layer; and conductors electrically connecting the connectionposts to the chiplet contact pads.

In certain embodiments, the chiplet contact pads are located on a sameside of the chiplet adjacent to the connection posts. In certainembodiments, the chiplet contact pads are located on a same side of thechiplet opposite to the connection posts.

In certain embodiments, the printable component includes a patternedelectrical connection layer between the connection posts and the chipletcontact pads.

In certain embodiments, the adhesive material underfills the volume andapplies compression between the printable component and the destinationsubstrate.

In certain embodiments, the connection post has a height that is greaterthan its base width, a base width that is greater than its peak width,or a base area that is greater than its peak area.

The present invention provides structures and methods that enable theconstruction of electrical interconnections between small integratedcircuits that are transfer printed on a destination substrate. Theelectrical interconnection process is simple and inexpensive requiringfewer process steps than known alternative methods.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, aspects, features, and advantages ofthe present disclosure will become more apparent and better understoodby referring to the following description taken in conjunction with theaccompanying drawings, in which:

FIG. 1 is a cross section of an embodiment of the present invention;

FIG. 2 is a cross section of another embodiment of the present inventionhaving components electrically connected in parallel;

FIG. 3 is a flow diagram illustrating a method of the present invention;

FIGS. 4 and 5 are cross sections illustrating steps in a method of thepresent invention;

FIG. 6 is a perspective illustrating steps in a method of the presentinvention;

FIGS. 7-8 are plan-view micrographs of components according toembodiments of the present invention;

FIG. 9 shows perspective micrographs of a connection post and acomponent according to embodiments of the present invention;

FIG. 10 shows cross-section micrographs of connection posts at tworesolutions according to embodiments of the present invention;

FIG. 11 shows plan-view micrographs of printed electrical connectionstructures and test results according to embodiments of the presentinvention;

FIGS. 12-13 are tables with values describing test results forembodiments of the present invention;

FIG. 14 shows plan-view micrographs of printed electrical connectionstructures according to an embodiment of the present invention; and

FIGS. 15 and 16 are cross sections of components according toembodiments of the present invention.

The features and advantages of the present disclosure will become moreapparent from the detailed description set forth below when taken inconjunction with the drawings, in which like reference charactersidentify corresponding elements throughout. In the drawings, likereference numbers generally indicate identical, functionally similar,and/or structurally similar elements. The figures are not drawn to scalesince the variation in size of various elements in the Figures is toogreat to permit depiction to scale.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a structure and method for electricallyconnecting relatively small electrical components such as integratedcircuit chiplets, LEDS, or electrical jumpers to a relatively largedestination substrate in an efficient and cost-effective way. Referringto FIGS. 1 and 2 in an embodiment of the present invention, a printedelectrical connection structure 50 includes a substrate 20 having one ormore electrical connection pads 22 formed on or in the substrate 20. Theconnection pads 22 can be, for example metal backplane contact pads madeusing photolithographic methods, tools, and materials. The substrate 20can be, for example, a printed circuit board or display substrate andcan include glass, polymer, plastic, resin, or fiber glass.

A printed component 10 has a circuit 80 and one or more connection posts16. The circuit 80 can be any structure responsive to, controlling, orconducting electrical energy supplied through the connection posts 16,such as an active circuit 80 including analog or digital circuits, atransistor, or an LED, or a passive circuit 80 including conductivewire, resistors, or capacitors. The printed component 10 can be anintegrated circuit or include an integrated circuit on a componentsubstrate separate from the integrated circuit and the substrate 80, forexample a dielectric substrate 86 as discussed below with respect toFIG. 15. Each connection post 16 is in electrical contact with aconnection pad 22. A resin 90 is disposed between and in contact with atleast a portion of the substrate 20 and the printed component 10, forexample in a patterned or an unpatterned layer. The resin 90 can be acurable polymer or epoxy. The resin 90 can be at least partly betweentwo or more connection posts 16 and can be an adhesive. In anembodiment, a plurality of connection posts 16 and connection pads 22are provided and each connection post 16 is electrically connected to atleast one connection pad 22. In another embodiment two or moreconnection posts 16 are electrically connected to a common connectionpad 22.

The resin 90 can have a disposed, deposited, or coated thickness that isless than a height of the connection posts 16, for example from theprinted component 10 in a direction orthogonal to a surface of theprinted component 10, as shown in FIGS. 1 and 2.

The resin 90 has a reflow temperature less than a cure temperature andthe resin 90 repeatedly flows at the reflow temperature when temperaturecycled between an operating temperature and the reflow temperature butwill no any longer flow after the resin 90 is exposed to the curetemperature. By providing a resin 90 that repeatedly flows when heatedto a reflow temperature less than a cure temperature, additional printedcomponents 10 can be sequentially disposed, for example bymicro-transfer printing, onto the substrate 20 and electricallyconnected to connection pads 22. The additional printed components 10can be provided to replace or serve in the place of defective printedcomponents 10 enabling improved yields for the printed electricalconnection structure 50.

In a further embodiment of the present invention illustrated in FIG. 2,a solder 24 or other electrically conductive material is disposed on theconnection post 16 (not shown), on the connection pad 22 (as shown) oron both the connection post 16 and the connection pad 22. The solder 24is in electrical contact with both the connection post 16 and theconnection pad 22. In various embodiments, the solder 24 has a meltingtemperature between the reflow temperature and the cure temperature, thesolder 24 has a melting temperature less than or equal to the reflowtemperature, or the solder 24 has a melting temperature greater than orequal to the cure temperature. The solder 24 can be patterned in a layerand can be disposed only on the connection pads 22 or connection posts16. The connection pad 22 can include metal and the solder 24 can besofter than the connection pad 22 metal. Alternatively or in addition,the connection pad 22 and the connection post 16 both include metal andthe metal of the connection pad 22 has a different hardness than themetal of the connection post 16. By providing elements with differenthardness, electrical connections are more readily formed by mechanicalpressure, such as that provided by micro-transfer printing.

As is also shown in FIG. 2, the reflowed resin 90 can substantially fillthe space (volume) between the printed component 10 and the substrate 20within the area defined or bounded by the connection posts 16, forexample a convex hull defined by the connection posts 16 over thesubstrate 20. The resin can be in contact with the dielectric substrate86 (FIG. 15, 16) of the printed component 10 and the substrate 20. Bysubstantially fill the space is meant that at least 50%, 60%, 70%, 80%,90%, 95%, or 99% of the volume is filled with resin 90. Alternatively,by substantially fill the space is meant that at least 50%, 60%, 70%,80%, 90%, 95%, or 99% of the area of both the substrate 20 and printedcomponent 10 defined or bounded by the connection posts 16, for examplea convex hull defined by the connection posts 16 over the substrate 20is covered by resin 90. Since, in an embodiment, the resin 90 can bedeposited at a thickness less than the height of the connection posts16, the space can be filled by reflowing the resin 90 so that it wicksover one or more surfaces of the printed component 10 between theprinted component 10 and the substrate 10 to fill the space and reducingthe amount of resin 90 in other areas over the substrate 20 that do nothave a printed component 10. Thus, the resin 90 can have a greaterthickness between the printed component 10 and the substrate 20 than atleast some other areas of the substrate 20, as can be seen by comparingthe thickness of the resin 90 in FIGS. 1 and 2.

Printed circuit board soldering techniques and materials can be used toprovide the solder 24 on the connection pads 22. For example, the solder24 can be a tin alloy. By providing the solder 24 with a desired meltingtemperature, the electrical connection between the connection pad 22 andthe connection post 16 can be enhanced by flowing the solder 24 so thatthe solder 24 wicks along the surfaces of the connection pad 22 and theconnection post 16.

In a further embodiment of the present invention and as illustrated inFIG. 2, the printed component 10 is a first printed component 10 and theone or more connection pads 22 are one or more first connection pads 22.A second printed component 12 has one or more second connection posts 17and each second connection post 17 is in electrical contact with thesecond connection pads 23, for example using wires 26. The wires 26 canbe made using printed circuit board or photolithographic methods,materials, and tools. The resin 90 is disposed between and in contactwith the substrate 20 and the second printed component 12 and the firstand second connection pads 22, 23 are electrically connected so that thefirst and second components 10, 12 are electrically connected inparallel. In an embodiment, the first component 10 is defective orotherwise fails to operate correctly and the second component 12functions in the place of the defective component 10.

FIGS. 15 and 16 are examples of micro-transfer printable components 10having a circuit 80 that includes a light-emitting diode (LED) 81 andelectrodes 84. Connection posts 16 are formed on a sacrificial portion88 of a patterned sacrificial layer 68 of a component source wafer 60 byetching a pyramidal structure into the sacrificial portion 88 and thenpatterning a conductive metal layer over the etched pyramidal structure.A dielectric substrate 86 such as silicon nitride is then deposited overthe sacrificial layer 68 and connection posts 16. The connection posts16 protrude from a post side 70 of the dielectric substrate 86. Vias 82are formed in a circuit side 72 opposing the post side 70 of thedielectric substrate 86 over portions of the patterned conductive metallayer forming the connection posts 16. The LED 81 is disposed on thecircuit side 72 of the dielectric substrate 86, for example bymicro-transfer printing the LED 81 from an LED source wafer to thedielectric substrate 86. Patterned dielectric structures 87 are formedto insulate the edges of the LED 81 and expose LED contacts 83 on acommon side of the LED 81 for supplying electrical power to the LED 81.The LED 81 can include multiple different layers such as conductionlayers and emission layers electrically connected to the electricalcontacts 83 and can emit light through an emission side of the LED 81opposite the common side or through the common side. As shown in FIG.16, the micro-transfer printable component 10 can have an encapsulationlayer 89 comprising a patterned second dielectric layer to protect thecomponent 10 and that can also serve as a tether 62 in addition to or inplace of the patterned dielectric substrate 86. Thus, the tether 62 canhave a thickness less than the thickness of the patterned dielectricsubstrate 86.

Patterned electrodes 84 are formed to electrically connect the LEDcontacts 83 to the connection posts 16 through the vias 82. Electricalpower supplied through the connection posts 16 causes the LED 81 to emitlight. Light can be emitted through the dielectric substrate 86 (asshown) or in a direction opposite to the dielectric substrate 86 (notshown), in which case the electrodes 84 do not cover the LED 81 commonside and a reflective layer can be located on the opposite side.Alternatively, the LED contacts 83 can be located on a common side ofthe LED 81 adjacent to the circuit side 72 of the dielectric substrate86 (not shown). Etching the patterned sacrificial portion 88 (formed onor in the surface of the component source wafer 60) forms a spacebeneath the component 10 with tethers 62 physically connecting thecomponent 10 to an anchor area 64 of the component source wafer 60 andreleases the micro-transfer printable component 10 from the componentsource wafer 60 so that the component 10 can be micro-transfer printed,for example to a destination substrate 20 (FIG. 1).

Forming the patterned sacrificial layer 68 and the pyramidal structures,patterning the conductive metal layer to form connection posts 16,depositing the dielectric substrate 86, forming the vias 82 in thedielectric substrate 86, patterning the dielectric structures 87, anddepositing and patterning the electrodes 84 can all be done usingphotolithographic material deposition and patterning techniques. Thesacrificial portion 88 of the patterned sacrificial layer 68 can be adesignated portion of an anisotropically etchable material such assilicon <1 0 0> oriented crystalline silicon or a different material.Alternatively, the dielectric substrate 86 can be an oxide layer such assilicon dioxide or a nitride layer such as silicon nitride. Theelectrodes 84 can be made of conventional electrically conductiveintegrated circuit materials, including aluminum, silver, titanium,copper, or other metals or metal alloys, as can the connection posts 16and connection pads 22.

Methods of forming micro-transfer printable structures are describedfurther, for example, in the paper AMOLED Displays usingTransfer-Printed Integrated Circuits. For a discussion of micro-transferprinting techniques see, U.S. Pat. Nos. 8,722,458, 7,622,367 and8,506,867, each of which is hereby incorporated by reference in itsentirety. Micro-transfer printing using compound micro-assemblystructures and methods can also be used with the present invention, forexample, as described in U.S. patent application Ser. No. 14/822,868,filed Aug. 10, 2015, entitled Compound Micro-Assembly Strategies andDevices, which is hereby also incorporated by reference in its entirety.In an embodiment, the component 10 is a compound micro-assembledstructure.

Referring to FIG. 3 according to an embodiment of the present invention,a method of making a printed electrical connection structure 50 includesproviding a substrate 20 having one or more electrical connection pads22 in step 100. In step 110 a resin 90 is disposed over the substrate 20over and in contact with at least a portion of the substrate 20, forexample on the connection pads 22 in a patterned or unpatterned layer.One or more components 10 are micro-transfer printed in step 120 from acomponent source wafer 60 to the destination substrate 20 as shown inFIGS. 4, 5, and 15, for example using a micro-transfer stamp 30 having apillar 32 in contact with each component 10. Each component 10 has oneor more connection posts 16 that are each positioned adjacent to acorresponding connection pad 22 so that the resin 90 is contacted to atleast a portion of the printed component 10, such as the connectionposts 16. A connection post 16 is adjacent to a connection pad 22 if noother connection pad 22 is closer to the connection post 16 than theconnection pad 22. The connection post 16 can be, but is notnecessarily, in contact with the connection pad 22 after themicro-transfer printing step 120, as shown in FIG. 5. The stamp 30 canthen be removed.

The printed electrical connection structure 50 is heated to the resin 90reflow temperature in step 130. In response, the resin 90 at leastpartially liquefies and wicks along surfaces with which it is incontact, such as the connection pads 22, the substrate 20, theconnection posts 16 and optionally portions of the printed component 10such as a semiconductor layer or component substrate such as thedielectric substrate 86 (FIG. 15, 16 below). The wicking process reducesthe space between the component 10 and the substrate 20 so that thecomponent 10 and the substrate 20 are drawn closer together incompression and the connection posts 16 and the connection pads 22 areforced into electrical contact (FIGS. 1 and 2). If a solder 24 layer ispresent (FIG. 2), the solder 24 can also flow and facilitate electricalconnection between the connection post 16 and the connection pad 22.Furthermore, the wicking process can increase the volume of material

The printed electrical connection structure 50 can be cooled to anoperating temperature, such as room temperature in step 140 andfunctionally tested (step 150), for example electrically tested byproviding or receiving electrical signals on the connection pads 22 thatoperate the printed component 10. If the components 10 can be testedwithout reflowing the resin 90, steps 130 and 140 can be skipped. In atest, for example, if the printed component 10 is an LED, light outputcan be observed or current through or voltage across the component 10 orcircuit 80 measured. If the printed component 10 passes the test, theprinted electrical connection structure 50 is heated to the curetemperature (step 160), the resin 90 is cured, and the printedelectrical connection structure 50 cooled again in step 170 (as in step140) and can be placed into operation.

If, however, the printed component 10 fails the test in step 150, it isoptionally removed in step 180. A second component 12 (FIG. 2) isprinted either in the location of the defective component 10 or ontosecond connection pads 23 that are electrically connected in parallel tothe first connection pads 22 of the defective component 10. The processcontinues as described above and the resin 90 is repeatedly refloweduntil a component 10 passes the test and a good component 10 isoperational. Once the resin 90 is cured, it can no longer reflow at thereflow temperature and the process is concluded.

According to an embodiment of the present invention, the micro-transferprinting step 120 transfers a plurality of components 10 in a singlestep. The components 10 can be tested in a common step 150 or all of thecomponents 10 tested before the additional components 10 are printed.Most of the components 10 are functional and the process of repeatedlyreflowing the resin 90 does not destroy the electrical connectionsbetween the connection posts 16 and the connection pad 22 of the goodcomponents 10 once the electrical connections between the connectionposts 16 and the connection pad 22 are made.

In a further embodiment of the present invention a conductor to thefailed or defective component 10 is optionally cut in step 190. In thecase in which the component 10 is electrically shorted, for example, itis useful to remove the defective component 10 from the electricalcircuit on the substrate 20 to prevent shorts between power signals andground signals and consequent power waste. This can be accomplished bycutting a power or ground line connection (e.g., wire 26) to thedefective or failed component 10. Lasers can pattern-wise cut electricalwires or traces on a backplane, substrate, or circuit boards toelectrically isolate defective components 10. The wires 26 can be cutbefore or after additional components 10 are micro-transfer printed instep 120 or before or after the resin 90 is cured in step 160.

FIG. 6 illustrates the methodology employed for makingpressure-concentrating conductive connection posts 16 within releasablecomponents 10 that can then be interconnected into systems usingelastomer stamp micro-transfer printing. Silicon wafers provide aconvenient template for making high-fidelity structures by wet-etching.Small inverted-pyramid cavities are formed in <100> oriented crystallinesilicon wafers using a silicon nitride hard-mask combined withanisotropic wet-etching with tetramethylammonium hydroxide (TMAH). Thesilicon nitride hard-mask is removed and a titanium-capped gold film(400 nm Au/50 nm Ti) is deposited and patterned. In one embodiment, thedeposited metal layers are designed to form redistribution structures,like the dog-bone pattern in FIG. 6(b), such that the component 10becomes a passive electrical “jumper”. A dielectric layer, in this casesilicon nitride deposited by plasma-enhanced chemical vapor deposition(PECVD), is deposited and patterned to form the mechanical backbone ofthe component 10. A second layer of PECVD silicon nitride is thendeposited and patterned to form the final component 10 structure,including anchors 64 and tethers 62. The silicon underneath thecomponent 10 is then removed by heated TMAH. The silicon directlyunderneath the anchor 64 region is not etched due to the nature of theanisotropic etch. Following this release etch process, the components 10remain fixed to the component source wafer 60 through the siliconnitride tethers 62 that exist between the component 10 and the anchor64.

FIG. 7 is an optical micrograph of undercut jumper components 10 ontheir silicon component source wafer 60. These components 10 have twoindependent metal dog-bones and each interconnection site has a 2×2array of the pressure-concentrating connection posts 16. The processfeatures wide design latitude. Examples of different jumper designs areshown in FIG. 8. In these images, the components 10 are flipped, withthe connection posts 16 pointing up. FIG. 8(d) shows a design thatincludes electrically insulating connection posts 16 which can be usefulfor mechanical supports or other non-electrical functions.

The released components 10 are now ready for integration into anon-native system using micro-transfer-printing. A viscoelasticelastomer stamp 30 is designed and fabricated to retrieve and transferarrays of the components 10 from their native component source wafer 60onto non-native application substrates 20. The stamp 30 mounts ontomotion-plus-optics machinery that can precisely control the stamp 30alignment and kinetics. During the printing, the machinery brings thestamp 30 into contact with the component source wafer 60, with opticalalignment performed before contact. FIG. 6(e) illustrates a singleelastomer pillar 32 laminated against a single component 10. Rapidupward movement of the print-head fractures the tether 62, transferringthe component 10 to the stamp 30. The populated stamp 30 then transitsto the destination substrate 20 and the components 10 are then alignedto the connection pads 22 and printed.

In a demonstration and according to an embodiment of the presentinvention, the destination substrates 20 are glass wafers with metalwires 26. The wires 26 on the glass wafers were patterned using lift-offtechniques and were comprised of electron-beam deposited Au (˜400 nm)with a Ti adhesion layer (˜100 nm). A layer of planarizing polymer resin90 (Dow Chemical Intervia 8023) such as an epoxy is applied to thedestination substrate 20 before printing. Here the thickness of thelayer of resin 90 was targeted to be 2.3 μm.

During printing, the populated stamp 30 contacts the components 10 tothe destination substrate 20, the connection pads 22, or the resin 90.During this step, the gold-coated connection posts 16 penetrate thelayer of resin 90, as seen in FIG. 2 discussed above and FIG. 10discussed below. The stamp 30 is able to apply additional penetrationforce by overdriving the stamp 30 in the z-direction past the originalcontact position. Controlled upward and lateral stamp motions allow thestamp 30 to transfer the components 10 to the destination substrate 20.

Following the transfer-printing, the samples undergo a hotplate bake(140° C.) where the polymer resin 90 flows under the component 10. Aftersystem assembly is complete the samples are fully cured at 175° C. forthree hours under flowing nitrogen.

This process uses stress-concentrating conductive connection posts 16 inconjunction with an underfill resin 90 to realize interconnected systemsthat do not require metallization steps after assembly. By underfill ismeant that the resin 90 does not fill the volume between the printedcomponent 10 and the destination substrate 20. Furthermore, with a heattreatment provided after disposing the resin 90, the resin 90 shrinksand provides compression between the printed component 10 and thedestination substrate 20 to further strengthen and make robust theelectrical connections between the connection posts 16 and the substrateconnection pads 22. FIG. 9(a) is a scanning electron micrograph, takenafter printing, showing the penetration of a connection post 16 into theplanarized resin 90 present on the destination substrate 20. Reliableelectrical connection typically requires reflow of the resin 90. Despitepenetration of the connection posts 16 into the resin 90 during theprinting step, some of the connection posts 16 and connection pads 22are not electrically connected until the reflow process (step 130, FIG.3). The post-print 140° C. treatment serves to reflow the polymer resin90. During reflow the polymer resin 90 wicks under the components 10, asshown in FIG. 9(b), and capillary action forces the connection posts 16of the components 10 into contact with the underlying metal traces ofthe connection pads 22. Measurement of the printed electrical connectionstructure 50 shows electrical continuity following the reflow process.After system assembly is complete, the underfill resin 90 is fully cured(step 160, FIG. 3). Thickness measurements (Nanospec 210) on blanketfilms of resin 90 indicate that unconstrained films of resin 90 shrinkin excess of 17% during the hard cure (step 160, FIG. 3). The shrinkageof the resin 90 pulls the component 10 toward the destination substrate20, which further drives the connection posts 16 into the connection pad22. Scanning electron micrographs of cross-sectioned samples are shownin FIG. 10. The higher magnification micrograph, FIG. 10(b), in thevicinity of the connection post 16 tip, reveals that the gold-coatedconnection post 16 makes intimate contact with and penetrates some depthinto the connection pad 22.

Embodiments of the present invention using electrically conductivejumper components 10 were tested. Daisy chain test circuits provide aconvenient platform to study the performance and reliability of thepressure-activated interconnecting components 10. FIG. 11(a) shows atarget landing site designed to accommodate a jumper component 10 withten independent interconnection sites. FIG. 11(b) shows the same siteafter integration of the jumper component 10. In this case, eachinterconnection is made by a single inverted-pyramid conductor. The 400nm thick gold landing traces are 10 μm wide. This embodimentdemonstrates an interconnect pitch of 15 μm in the y-direction.

The daisy chain is designed to accept a 20×20 array of jumper components10 in a single transfer-print operation. The pitch between the printedjumper component 10 is 500 μm. A micrograph of the full daisy-chain testvehicle is shown in FIG. 11(c). Two-wire resistance measurements with anohmmeter (Fluke 45) are shown in FIG. 11(d) and the table of FIG. 12. Inthis example, the resistance per jumper component 10 is approximately5.7 Ohms. These printed electrical connection structures 50 have tenindependent interconnects, which indicates a resistance per interconnectof 0.57 Ohms, including the resistance of the wires 26 and the contactresistance of the interconnection.

Fully interconnected daisy chains have been subjected to numerousreliability tests. The table of FIG. 13 shows the resistance changes ofa full chain with 1120 interconnections that was subjected to thermalshock and pressure humidity testing. Elevated temperature and mechanicaltests were also performed. The interconnected devices exhibit remarkableresilience to harsh stressors. This is likely due in part to themicro-scale nature of the integrated elements and also to the inherentability for this process to underfill the components 10.

A compelling attribute of the present invention is the ability to repairdefects after system-level testing (step 150, FIG. 3). As discussedabove, the printed electrical connection structures 50 are electricallyinterconnected and testable following the reflow (step 130, FIG. 3) ofthe underfill resin 90. Even though the resin 90 flows, it remainslargely unchanged after returning to room temperature. Thicknessmeasurements on blanket films of polymer resin 90 show a ˜2% thicknessreduction following a reflow process (140° C., 4 min), which is mostlyattributed to further evaporation of the carrier solvent. The polymerresin-coated destination substrate 20 remains receptive to repeatedtransfer-prints (step 120, FIG. 3) following a first print, reflow andtest sequence, see FIG. 14(a). An example of a repaired series chain isshown in FIG. 14(b).

Another important attribute of the present invention is the ability toefficiently provide a micro-transfer printed display, test, and repairprocess. A very large number of micro-LEDs can be micro-transfer printedin the initial construction process (e.g., more than 1,000 micro-LEDsper print, more than 10,000 micro-LEDs per print, more than 50,000micro-LEDs per print, or more than 100,000 micro-LEDs per print) so thatlarge, high-resolution displays can be printed in only a few minutes.All of the devices can be electrically interconnected in a commonphotolithographic step (or as part of the micro-transfer printingprocess using connection posts) and tested in a common test step, forexample using an optical test fixture with image analysis. Faileddevices can then be removed or disconnected using a high-speed lasersystem or repair devices installed using a single or multiple print-headmicro-LED micro-transfer printing system.

Conventional silicon micromachining by anisotropic wet-etching providesconvenient routes to the fabrication of metal-coated pyramidalconnection posts 16 that can be integrated within fully undercutmicro-transfer-print compatible components 10. The soft elastomer stamps30 employed in micro-transfer-printing can provide the down forcerequired for the connection posts 16 to penetrate into a resin 90underfill. Reflow of the underfill layer of polymer resin 90 drivescompletion of the electrical interconnection through capillary forces. Asimple demonstration showed how the interconnection strategy can be usedfor repair after electrical test of the printed electricalinterconnection structure 50.

Some applications of the present invention can benefit from strategiesthat complete the device interconnections in combination with theassembly process. Examples include large-format or mechanically flexibleapplications where it is not convenient to perform traditionallithography and metallization processes following the system assembly.

The component 10 can be an active component, for example including oneor more active elements such as electronic transistors or diodes orlight-emitting diodes and photodiodes that produce an electrical currentin response to ambient light. Alternatively, the component 10 can be apassive component, for example including one or more passive elementssuch as resistors, capacitors, or conductors. In another embodiment, thecomponent 10 is a compound component 10 that includes both active andpassive elements. The component 10 can be or include a semiconductordevice having one or more semiconductor layers, such as an integratedcircuit. The component 10 can be or include an unpackaged die. In yetanother embodiment, the component 10 is a compound element having aplurality of active or passive elements, such as multiple semiconductordevices with separate substrates, each with one or more active elementsor passive elements, or both. In certain embodiments, the plurality ofelements is disposed and interconnected on a compound componentsubstrate (e.g., dielectric substrate 86) separate and independent fromthe substrates of any semiconductor devices. The compound component 10can be micro transfer printed itself after the elements have beenarranged thereon. The components 10 can be electronic processors,controllers, drivers, light-emitting diodes, photodiodes, light-controldevices, or light-management devices.

The components 10 made by methods of the present invention can includeor be a variety of chiplets having semiconductor structures, including adiode, a light-emitting diode (LED), a transistor, or a laser. Chipletsare small integrated circuits and can be unpackaged dies released from asource wafer and can be micro transfer printed. Chiplets can have atleast one of a width, length, and height from 2 to 5 μm, 5 to 10 μm, 10to 20 μm, or 20 to 50 μm. Chiplets can have a doped or undopedsemiconductor substrate thickness of 2 to 5 μm, 5 to 10 μm, 10 to 20 μm,or 20 to 50 μm or can have glass, polymer, oxide, or nitride substrates.The chiplet or components 10 can include micro-light-emitting diodeswith a length greater than width, for example having an aspect ratiogreater than or equal to 2, 4, 8, 10, 20, or 50 and component contactpads (e.g., contacts 83) that are adjacent to the ends of the printablesemiconductor components 10 along the length of the printablesemiconductor components 10. This structure enables low-precisionmanufacturing processes to electrically connect wires 26 to thecomponent contact pads without creating registration problems andpossible unwanted electrical shorts or opens.

The components 10 can include active elements such as electroniccircuits 80 formed using lithographic processes and can include passiveelements such as electrical connections, e.g., wires, connecting thecomponent contact pads and connection posts 16. In certain embodiments,the component contact pads 83 are planar electrical connections. Suchcomponent contact pads can be formed from metals such as aluminum orpolysilicon using various masking and deposition processes.

In some embodiments of the present invention, the components 10 are orinclude small integrated circuits, for example chiplets, having a thinsubstrate with a thickness of only a few microns, for example less thanor equal to 25 microns, less than or equal to 15 microns, or less thanor equal to 10 microns, and a width or length of 5-10 microns, 10-50microns, 50-100 microns, or 100-1000 microns. Such chiplet or component10 can be made in a source semiconductor wafer (e.g., a silicon or GaNwafer) having a process side and a back side used to handle andtransport the wafer. Chiplets, components 10, or portions of components10 are formed using lithographic processes in an active layer on or inthe process side of the source wafer. An empty release layer space isformed beneath the components 10 with tethers connecting the components10 to the source wafer in such a way that pressure applied against thecomponents 10 breaks the tethers to release the components 10 from thesource wafer. Lithographic processes can be used to form components 10in a source wafer, for example transistors, wires, and capacitors.

According to various embodiments of the present invention, the componentsource wafer 60 can be provided with the components 10, patternedsacrificial layer 68, tethers 62, and connection posts 16 alreadyformed, or they can be constructed as part of the process of the presentinvention.

Connection posts 16 are electrical connections formed on the component10 that extend generally perpendicular to a surface of the component 10.Such connection posts 16 can be formed from metals such as aluminum,titanium, tungsten, copper, silver, gold, or other conductive metals.The connection posts 16 can be formed by repeated masking and depositionprocesses that build up three-dimensional structures. Alternatively, theconnection posts 16 can be made by etching one or more layers of metalevaporated or sputtered on the process side of the component 10. Suchstructures can also be made by forming a layer above or below thecomponent 10 surface (e.g., the patterned sacrificial layer 68), etchinga well into the layer, filling it with a conductive material such asmetal, and then removing the layer. The connection posts 16 are thenelectrically connected to contact pads in the component 10. In someembodiments, the connection posts 16 are made of one or more highelastic modulus metals, such as tungsten. As used herein, a high elasticmodulus is an elastic modulus sufficient to maintain the function andstructure of the connection post 16 when pressed into a substrateconnection pad 22.

The connection posts 16 can have a variety of aspect ratios andtypically have a peak area smaller than a base area. The connectionposts 16 can have a sharp point for embedding in or piercing thesubstrate connection pads 22. The connection posts 16 can have a basewidth representing a planar dimension of the connection post 16 on theprocess side and a height representing the extent of the connection post16 from the process side to the peak of the connection post 16. The peakof the connection post 16 can have a width less than base width and, inan embodiment, approaches zero so that the connection post 16 has asharp point. The base of the connection post 16 can have a base area incontact with the process side and a peak area smaller than the basearea. The connection post 16 can also have a height greater than a basedimension.

In an embodiment, the connection post 16 is softer than the connectionpad 22 so that the connection post 16 can crumple when the connectionpost 16 is under mechanical pressure. Alternatively, the connection pad22 is softer than the connection post 16 so that it deforms before theconnection post 16 when under mechanical pressure. By deform is meantthat the connection posts 16 or the substrate connection pads 22 changeshape as a consequence of the transfer printing.

The substrate connection pads 22 can be made of a relatively soft metal,such as tin, solder, or tin-based solder, to assist in forming goodelectrical contact with the connection posts 16 and adhesion with thecomponents 10. As used herein, a soft metal may refer to a metal intowhich a connection post 16 can be pressed to form an electricalconnection between the connection post 16 and the substrate connectionpad 22. In this arrangement, the substrate connection pad 22 canplastically deform and flow under mechanical pressure to provide a goodelectrical connection between the connection post 16 and the substrateconnection pad 22. In another embodiment of the present invention, theconnection posts 16 can include a soft metal and the substrateconnection pads 22 include a high elastic modulus metal. In thisarrangement, the connection posts 16 can plastically deform and flowunder mechanical pressure to provide a good electrical connectionbetween the connection post 16 and the substrate connection pads 22.

The layer of resin 90 can be cured to more firmly adhere the components10 to the destination substrate 20 and maintain a robust electricalconnection between the connection posts 16 and substrate connection pads22 in the presence of mechanical stress. The adhesive resin 90 canundergo some shrinkage during the curing process that can furtherstrengthen the electrical connectivity and adhesion between theconnection post 16 and the substrate connection pads 22.

In alternative embodiments of the present invention, the connectionposts 16 of the components 10 are in contact with, are embedded in, orpierce the substrate connection pads 22 of the destination substrate 20.Alternatively, deformation or crumpling of the connection pads 22 orconnection posts 16 can improve the electrical connection between theconnection posts 16 and the substrate connection pads 22 by increasingthe surface area that is in contact with the connection posts 16 and thesubstrate connection pads 22. To facilitate deformation, in anembodiment the two or more connection posts 16 have a composition softerthan that of the substrate connection pads 22 or the substrateconnection pads 22 have a composition softer than the connection posts16.

A compliant polymer layer can be formed beneath the substrate connectionpads 22 to facilitate the mechanical contact made when the connectionposts 16 are embedded in the backplane connection pads 22. For example,a metal or metal alloy containing as gold, tin, silver, or aluminum, canbe formed over a polymer layer or a polymer layer coated over a metal ormetal alloy containing gold, tin, silver, or aluminum. The compliantpolymer layer can also serve to adhere the connection posts 16 to thesubstrate connection pads 22.

In an embodiment two or more connection posts 16 are electricallyshorted in a component 10 and electrically connected to a commonsubstrate connection pad 22. Such redundant electrical connectionsreduce contact failures between the connection posts 16 and thesubstrate connection pads 22, for example as shown in FIG. 8.

The spatial distribution of the components 10 is a matter of designchoice for the end product desired. In one embodiment of the presentinvention, all of the components 10 in a component source wafer arrayare transferred to the stamp 30. In another embodiment, a subset of thecomponents 10 in the component source wafer array is transferred. Byvarying the number and arrangement of pillars 32 on transfer stamps 30,the distribution of components 10 on the pillars 32 of the transferstamp 30 can be likewise varied, as can the distribution of thecomponents 10 on the destination substrate 20.

In an embodiment of the present invention, the component 10 is alight-emitting component 10 that emits light in a direction opposite tothe connection posts 16 or in a direction of the connection posts 16.

According to one embodiment of the present invention, a source wafer canbe provided with components 10 and connection posts 16 already formed.Alternatively, an unprocessed source wafer can be provided and thecomponents 10 formed on the component source wafer 60. An unprocessedsource wafer is a substrate that does not yet include components 10. Theunprocessed source wafer can have other processing steps completed, forexample, cleaning, deposition of material layers, or heat or chemicaltreatments. Components 10 are formed, for example usingphoto-lithographic processes including forming masks over the sourcewafer, etching materials, removing masks, and depositing materials.

Components 10 can be or include small electronic integrated circuits,inorganic light-emitting diodes, or electrical connectors (jumpers) forexample, having a size of about 5 microns to about 5000 microns in adimension. The electronic circuits 80 can include semiconductormaterials (for example inorganic materials such as silicon or galliumarsenide, or inorganic materials) having various structures, includingcrystalline, microcrystalline, polycrystalline, or amorphous structures.In another embodiment, the components 10 are passive, for exampleincluding a conductor that, when used in a printed electrical connectionstructure 50 serves to electrically connect one conductor (e.g., asubstrate connection pad 22) to another, forming a jumper. Thecomponents 10 can also include insulating layers and structures such assilicon dioxide, nitride, and passivation layers and conductive layersor structures including wires made of aluminum, titanium, silver, orgold that form an electronic circuit 80. Large numbers of such smallintegrated circuits can be formed on a single source wafer. Thecomponents 10 are typically packed as closely as possible to use thesurface area of the source wafer as efficiently as possible.

In some embodiments, the components 10 are or include small integratedcircuits formed in a semiconductor wafer, for example gallium arsenideor silicon, which can have a crystalline structure. Processingtechnologies for these materials typically employ high heat and reactivechemicals. However, by employing transfer technologies that do notstress the component 10 or substrate materials, more benignenvironmental conditions can be used compared to thin-film manufacturingprocesses. Thus, the present invention has an advantage in that flexiblesubstrates, such as polymeric substrates, that are intolerant of extremeprocessing conditions (e.g. heat, chemical, or mechanical processes) canbe employed for the destination substrates 20. Furthermore, it has beendemonstrated that crystalline silicon substrates have strong mechanicalproperties and, in small sizes, can be relatively flexible and tolerantof mechanical stress. This is particularly true for substrates having5-micron, 10-micron, 20-micron, 50-micron, or even 100-micronthicknesses. Alternatively, the components 10 can be formed in amicrocrystalline, polycrystalline, or amorphous semiconductor layer.

The components 10 can be constructed using foundry fabricationprocesses. Layers of materials can be used, including materials such asmetals, oxides, nitrides and other materials. Each component 10 can beor include a complete semiconductor integrated circuit and can include,for example, transistors, diodes or light-emitting diodes. Thecomponents 10 can have different sizes, for example, 1000 square micronsor 10,000 square microns, 100,000 square microns, or 1 square mm, orlarger, and can have variable aspect ratios, for example 1:1, 2:1, 5:1,or 10:1. The components 10 can be rectangular or can have other shapes.

Embodiments of the present invention provide advantages over otherprinting methods described in the prior art. By employing connectionposts 16 on components 10 and a printing method that provides components10 on a destination substrate 20 connection posts 16 in contact with thedestination substrate 20 or layers formed on the destination substrate20, a low-cost method for printing components 10 in large quantitiesover a destination substrate 20 is provided. Furthermore, additionalprocess steps for electrically connecting the components 10 to thedestination substrate 20 are obviated.

The source wafer and components 10, transfer stamp 30, and destinationsubstrate 20 can be made separately and at different times or indifferent temporal orders or locations and provided in various processstates.

The method of the present invention can be iteratively applied to asingle or multiple destination substrates 20. By repeatedly transferringsub-arrays of components 10 from a transfer stamp 30 to a destinationsubstrate 20 and relatively moving the transfer stamp 30 and destinationsubstrates 20 between stamping operations by a distance equal to thespacing of the selected components 10 in the transferred sub-arraybetween each transfer of components 10, an array of components 10 formedat a high density on a component source wafer 60 can be transferred to adestination substrate 20 at a much lower density. In practice, thecomponent source wafer 60 is likely to be expensive, and formingcomponents 10 with a high density on the component source wafer 60 willreduce the cost of the components 10, especially as compared to formingcircuits on the destination substrate 20. Transferring the components 10to a lower-density destination substrate 20 can be used, for example, ifthe components 10 manage elements distributed over the destinationsubstrate 20, for example in a display, digital radiographic plate, orphotovoltaic system.

In particular, in the case wherein the active component 10 is orincludes an integrated circuit formed in a crystalline semiconductormaterial, the integrated circuit substrate provides sufficient cohesion,strength, and flexibility that it can adhere to the destinationsubstrate 20 without breaking as the transfer stamp 30 is removed.

In comparison to thin-film manufacturing methods, using denselypopulated component source wafers 60 and transferring components 10 to adestination substrate 20 that requires only a sparse array of components10 located thereon does not waste or require active layer material on adestination substrate 20. The present invention can also be used intransferring components 10 made with or including crystallinesemiconductor materials that have higher performance than thin-filmactive circuits. Furthermore, the flatness, smoothness, chemicalstability, and heat stability requirements for a destination substrate20 used in embodiments of the present invention may be reduced becausethe adhesion and transfer process is not substantially limited by thematerial properties of the destination substrate 20. Manufacturing andmaterial costs may be reduced because of high utilization rates of moreexpensive materials (e.g., the source substrate) and reduced materialand processing requirements for the destination substrate 20.

As is understood by those skilled in the art, the terms “over” and“under” are relative terms and can be interchanged in reference todifferent orientations of the layers, elements, and substrates includedin the present invention. For example, a first layer on a second layer,in some implementations means a first layer directly on and in contactwith a second layer. In other implementations a first layer on a secondlayer includes a first layer and a second layer with another layertherebetween.

Having described certain implementations of embodiments, it will nowbecome apparent to one of skill in the art that other implementationsincorporating the concepts of the disclosure may be used. Therefore, thedisclosure should not be limited to certain implementations, but rathershould be limited only by the spirit and scope of the following claims.

Throughout the description, where apparatus and systems are described ashaving, including, or comprising specific elements, or where processesand methods are described as having, including, or comprising specificsteps, it is contemplated that, additionally, there are apparatus, andsystems of the disclosed technology that consist essentially of, orconsist of, the recited elements, and that there are processes andmethods according to the disclosed technology that consist essentiallyof, or consist of, the recited processing steps.

It should be understood that the order of steps or order for performingcertain action is immaterial so long as the disclosed technology remainsoperable. Moreover, two or more steps or actions in some circumstancescan be conducted simultaneously. The invention has been described indetail with particular reference to certain embodiments thereof, but itwill be understood that variations and modifications can be effectedwithin the spirit and scope of the invention.

PARTS LIST

-   10 component/first component/printed component-   12 second component-   16 connection post/first connection post-   17 second connection post-   20 substrate/destination substrate-   22 connection pad/first connection pad/substrate connection pad-   23 second connection pad-   24 solder-   26 wire-   30 stamp/transfer stamp-   32 pillars-   50 printed electrical connection structure-   60 component source wafer-   62 tether-   64 anchor-   68 patterned sacrificial layer-   70 post side-   72 circuit side-   80 circuit-   81 light-emitting diode-   82 via-   83 contact-   84 electrode-   86 dielectric substrate/component substrate-   87 patterned dielectric-   88 sacrificial portion-   89 encapsulation layer/second dielectric layer-   90 resin-   100 provide destination substrate with connection pads step-   110 coat resin on destination substrate step-   120 micro-transfer print component from source wafer to destination    substrate step-   130 heat printed structure to reflow temperature step-   140 cool printed structure to operating temperature step-   150 test printed structure step-   160 heat printed structure to cure temperature step-   170 cool printed structure to operating temperature step-   180 optional remove printed structure step-   190 optional cut conductor to failed component step

1. A printed electrical connection structure, comprising: a substratehaving one or more electrical connection pads; a printed componenthaving one or more connection posts, each connection post in electricalcontact with a connection pad; and a resin disposed between and incontact with the substrate and the printed component, the resin having areflow temperature less than a cure temperature, wherein the resinrepeatedly flows at the reflow temperature when temperature cycledbetween an operating temperature and the reflow temperature but does notflow after the resin is exposed to the cure temperature.
 2. The printedstructure of claim 1, comprising a solder disposed on the connectionpost, on the connection pad or on both the connection post and theconnection pad. 3-9. (canceled)
 10. The printed structure of claim 1,wherein the printed component is a first printed component, the one ormore connection pads are one or more first connection pads, andcomprising a second printed component having one or more secondconnection posts, each second connection post in electrical contact withthe second connection pads, the resin is disposed between and in contactwith the substrate and the second printed component, and the first andsecond connection pads are electrically connected so that the first andsecond components are electrically connected in parallel.
 11. (canceled)12. A method of making a printed structure, comprising: providing asubstrate having one or more electrical connection pads; disposing aresin layer over and in contact with at least a portion of thesubstrate; micro-transfer printing a component from a component sourcewafer to the substrate, the component having one or more connectionposts that are each positioned adjacent to a corresponding connectionpad, so that the resin contacts at least a portion of the printedcomponent.
 13. The method of claim 12, wherein the resin has a reflowtemperature less than a cure temperature, wherein the resin repeatedlyflows at a first rate at the reflow temperature when temperature cycledbetween an operating temperature and the reflow temperature but does notflow after the resin is exposed to a cure temperature.
 14. The method ofclaim 13, comprising testing the component.
 15. The method of claim 13,comprising heating the resin to the reflow temperature and electricallycontacting each connection post to the corresponding connection pad. 16.The method of claim 15, comprising cooling the structure after heatingthe resin to the reflow temperature.
 17. The method of claim 16,comprising reheating the structure to the reflow temperature aftercooling the structure.
 18. The method of claim 16, comprising coolingthe structure after reheating the resin to the reflow temperature. 19.The method of claim 18, comprising heating the structure to the curetemperature after reheating the resin to the reflow temperature.
 20. Themethod of claim 17, comprising testing the structure after reheating theresin to the reflow temperature.
 21. The method of claim 15, comprisingtesting the structure after heating the resin to the reflow temperature.22. The method of claim 12, comprising providing a layer of solder overat least the one or more connection pads, the solder having a meltingtemperature. 23-25. (canceled)
 26. The method of claim 12, comprisingproviding an electrical conductor electrically connected to one of theconnection pads, and cutting the electrical conductor.
 27. The method ofclaim 12, wherein the component is a first component, the one or moreconnection pads are one or more first connection pads, and comprisingproviding one or more second connection pads electrically connected inparallel with the one or more first connection pads.
 28. The method ofclaim 27, comprising determining that the first component failed thetest.
 29. The method of claim 28, comprising providing an electricalconductor electrically connected to one of the connection pads, andcutting the electrical conductor. 30-34. (canceled)
 35. A micro-transferprintable component, comprising: a dielectric substrate having a postside and a circuit side; one or more electrically conductive connectionposts protruding from the post side of the dielectric substrate; acircuit disposed on the circuit side of the dielectric substrate; and anelectrode electrically connecting each of the connection posts to thecircuit.
 36. The micro-transfer printable component of claim 35,comprising a via corresponding to each connection post, the viaextending from the circuit side of the dielectric substrate to a portionof the corresponding connection post, the electrode extending into thevia to electrically connect the circuit to the corresponding connectionpost. 37-50. (canceled)